The present invention relates to recording and editing of magnetic video tape color recordings and, in particular, to an improved system for insuring proper color burst phase matching during recording.
Electronic splicing of television magnetic tape containing composite NTSC or PAL color signals is complicated by the nature of the television signal itself, and by the manner in which the standard video tape recorder (VTR) processes the signal on playback.
In the NTSC system, the color burst phase differs 180.degree. from one line of video to the next. This is because the color subcarrier frequency is a 455/2 multiple of the horizontal scanning frequency. That is, for each two lines of video, the subcarrier is able to complete a whole number of cycles. Consequently, succeeding frames of NTSC video have opposite burst phases, when compared on a line-for-line basis, and four television fields must occur before the unmodulated subcarrier exactly repeats itself.
In the PAL system, the combination of the 90.degree. alternating burst and the 90.degree. dropping back of the burst phase causes consecutive pairs of lines to have the same burst phase, and adjacent pairs to be 180.degree. out of phase. Because each frame has an odd number of lines, 625, four PAL video frames, eight video fields, are required before the burst phase repeats itself, line-for-line, within a frame.
If a continuous signal is to be reproduced, splices must join succeeding color frames. If they do not, there will be an abrupt 180.degree. shift of burst and chroma at the splice, which can adversely affect, for example, some modes of editing.
Thus, for either the NTSC or PAL systems, when new video signals are to be recorded on a VTR following a previously recorded segment, the VTR has a 50--50 chance of locking to the correct color frame. This is discussed in greater detail in "The Problems of Splicing and Editing Color Video Magnetic Tape", by C. A. Anderson, IEEE Transactions on Broadcasting, Vol. BC-15, No. 3, September 1969, pp. 59-61.
Thus, one-half of the time, the VTR locks up with a frame of the video which has its color burst 180.degree. out of phase with that which was previously recorded. For an ordinary, uninterrupted replay, this presents no problem. But, if a number of video segments are mixed and sequentially recorded, serious difficulties are encountered. As the video head moves from old recording to new recording during replay of the edited tape, a 180.degree. phase shift is encountered with respect to sync at the edit point, and the VTR time base correction circuits, to compensate, insert or remove a 140 ns delay, causing the picture to jump sideways.
This effect is not disturbing if such edits are only occasional, particularly if the scene content changes. But if there is a series of closely spaced splices or if there is animation, the picture continually hops back and forth. At worst, a complete break-up of the picture occurs.
Several approaches have been suggested or implemented to overcome this color phase matching problem. Several of these are discussed in the Anderson article referred to above. The way which is most commonly used involves changing the edit point by one frame, in the case of NTSC, or two frames for PAL, if improper color phase matching occurs. This technique involves the following steps.
First, when a video signal is to be recorded by the VTR, it is provided to the VTR in the usual manner and the conventional synchronization process begins. The sync pulses from the recording tape are compared with the plant reference sync pulses. Any phase deviation results in regulation of the VTR capstan tape drive to regulate the tape speed so that the tape sync pulses are in phase with the reference sync pulses.
Next, the tape color burst signal is compared with the plant color burst reference, a 3.58 MHz subcarrier. Since the phase of the tape color burst varies because of time-base instabilities, a delay is introduced or deleted to compensate for these time-base instabilities, so that the tape color burst is synchronized with the 3.58 MHz plant reference.
At this point, there is a 50--50 chance that the VTR has locked to the correct color frame, as explained previously. A signal is developed to indicate which of these two conditions occurs. If there is a phase mismatch, the tape capstan drive speed is altered so that the VTR tape "slips" one frame relative to the plant reference, and the entire synchronization process is repeated, but with proper color framing.
This technique has a number of significant disadvantages. First, when editing, all playback VTR's, i.e. those VTR's containing the scenes to be edited, are normally slaved to the plant reference signal and the color burst is automatically in phase with the plant color burst. The effect of causing the record VTR tape to slip back relative to the plant color reference to bring about proper color framing is that it also slips back relative to the playback VTR's and so the edit point is shifted by one frame, in the case of NTSC, or two frames in the case of PAL. Many editors, concerned with the aesthetics of the composite edited tape, object to alteration of edit points, even if it is only by one frame.
Secondly, this approach relies upon some method of detecting, at the beginning of each recording, the color phase to see if a frame slip is required. For example, sensing a phase error voltage or sync timing signal is required, which experience has shown requires frequent, critical adjustment.
Third, the "detect-and-bump" cycle, during the 50 percent of the times when color framing is required, introduces a 4-5 second delay into the editing sequence, and the worst case condition must be allowed for in judging roll timing.
Fourth, the color phase, the very thing which is sensed immediately after initial synchronization is most disturbed at that very moment in time. Therefore, poor editing results frequently occur where a series of closely spaced edits occur. The undesirable alternative is to sequence or space the edit points.
Examples of this type of system are the Ampex Color Framing Accessory and the device described in U.S. Pat. No. 3,594,498.